I had the same problem. I went ot see a local cycling coach and he found that my saddle height was too low so my knees were bottoming out and shooting outwards causing the pain. I am now pain free and riding much stronger.
Remember that if you are riding an average of 200 miles a week at an average of 90rpm and 18mph, you are doing about 3120000 revolutions per year. Think about the effect it has on your knees when you are not set up correctly.
Spend the money on a decent bike fit.